my %isis_adjacencies;

for my $R2 (@array) {
    push @{$isis_adjacencies{$R2}}, $R2;
}

##</code><code>##

$ perl -MData::Dump -e 'my %x; my @y = qw{a b c}; push @{$x{$_}}, $_ for @y; dd \%x'
{ a => ["a"], b => ["b"], c => ["c"] }